It has previously been proposed to monitor the inking of the plate cylinder of printing machines by checking the distribution of inking in circumferential zones, located adjacent each other, using a densitometric apparatus (see the referenced French Pat. No. 1,519,883). The densitometric measuring apparatus checks the ink density during rotation of the plate cylinder with respect to the entire circumference thereof. It has been found that the measured results do not necessarily correspond to the actual inking. The blanket cylinder, in operation, will pick up dust, fluff, or other contaminants from the printing substrate, typically paper. The contaminants are transferred in the course of operation from the blanket cylinder to the plate cylinder, and the ink measuring apparatus will then respond not only to the actual inking, but to the ink as contaminated. The output derived from the densitometers, then, could be erroneous, and non-representative of the actual inking.